1. Field of the Invention
The present invention relates to apparatus and methods for economically forming precision molded articles, in particular hard molds for plastic injection molding or die casting, from mixes of refractory particles and heat fugitive organic binders.
2. Description of the Prior Art
The present invention has broad utility and can be used to form many kinds of molded refractory articles. For illustrative purposes, the invention and its background will be described in the context of the fabrication of hard molds for use in plastic injection molding.
Techniques for molding sintered metal articles from mixes of refractory particles or powders and heat fugitive organic binders have long been known. Such techniques can be used, for example, to make sintered metal core and cavity inserts for durable injection molds capable of producing millions of plastic injection molded parts. In a typical process currently in use, shown schematically in FIG. 1, a flexible molding material such as RTV (room temperature vulcanized) silicone rubber 200 is poured around a master pattern 202 in a frame 204 and allowed to cure, forming an interim mold pattern 206. Alternatively, the process may begin with pattern 206 as the master pattern. In either case, it is typical for the master pattern to be formed using stereolithography. Once the RTV has cured, the master pattern is separated from the interim RTV mold and an intermediate mold 208 is formed by pouring a second RTV material in and around the interim RTV mold. This second RTV material is cured, and the interim RTV 206 removed to leave a cavity 210 in mold 208. A mixture of refractory metal particles (for example, A.sub.6 tool steel particles and tungsten carbide particles) and a heat fugitive organic binder 212 (for example, a thermoplastic polymer or catalyzed epoxy liquid) is poured into the intermediate RTV mold. The epoxy is cured to hold the particles together. The thermoplastic polymer may be heated slightly to cause melting and then cooled or it may be compacted to create adhesion. The refractory particles mixed with the cured epoxy or solidified polymer, referred to as a green article, is removed from the RTV mold and heated to melt, or burn off, the binder and tack the refractory particles together and sinter the refractory particles to form a porous sintered article. The sintered refractory article is then infiltrated with a metallic infiltrant, such as copper. The metallic infiltrant has a melting point lower than that of the porous sintered article. The result is a mold of a metal composite refractory article (MCRA) 213 of shape and dimensions suitable for forming, by injection molding therein, a polymeric structure 214 duplicating the master, the mold having both high hardness and high impact strength. The technique is particularly useful for producing tooling for a plastic injection molding.
Production and use of tooling formed using powder metal technology are known in the art. Examples of techniques for such production and use are disclosed in the following U.S. Patents that are each herein incorporated by reference in their entirety:
(1) U.S. Pat. No. 3,823,002, entitled "Precision Molded Refractory Articles," issued July 1974 to Kirby, et al.
(2) U.S. Pat. No. 3,929,476, entitled "Precision Molded Refractory Articles and Method of Making," issued December 1975 to Kirby, et al.
(3) U.S. Pat. No. 4,327,156 entitled "Infiltrated Powdered Metal Composites Article," issued April 1982 to Dillon, et al.
(4) U.S. Pat. No. 4,373,127, entitled "EDM Electrodes," issued February 1983 to Hasket, et al.
(5) U.S. Pat. No. 4,431,449, entitled "Infiltrated Molded Articles of Spherical Non-Refractory Metal Powders," issued February 1984 to Dillon, et al.
(6) U.S. Pat. No. 4,455,354, entitled "Dimensionally-Controlled Cobalt Containing Precision Molded Metal Article," issued June 1984 to Dillon, et al.
(7) U.S. Pat. No. 4,469,654, entitled "EDM Electrodes," issued September 1984 to Hasket, et al.
(8) U.S. Pat. No. 4,491,558, entitled "Austenitic Manganese Steel Containing Composite Article," issued January 1985, to Gardner.
(9) U.S. Pat. No. 4,554,218, entitled "Infiltrated Powdered Metal Composite Article," issued November 1985, to Gardner, et al.
(10) U.S. Pat. No. 5,507,336, entitled "Method of Constructing Fully Dense Metal Molds and Parts," issued to Tobin.
Although the described technique is relatively straightforward, it has several shortcomings in the fabrication of molds, tooling, usable articles, or full size prototype devices:
First, the effective cost per cubic inch of an MCRA is many times greater than that of a device machined from conventional tool steel. For especially small parts (for example, 1-3 cu. in.) this difference in material cost is not significant relative to set-up charges or incremental charges associated with increased speed of delivery. However, for larger pieces (for example, 10 cu. in. and greater), the material cost penalty for the entire tool (core and cavity) becomes substantial and the process becomes far less economically attractive. In particular, the mixture of refractory particles and binder can be a significant part of the costs to prepare a final part. Additionally, since the master has to be removed from within the RTV mold, certain parts, particularly masters with undercuts and complicated geometries cannot be readily reproduced and the RTV mold can be torn during the process of removing it from the master.
Secondly, the green article shrinks appreciably during sintering and infiltration. Such shrinkage may be nonuniform when sections of significantly different thicknesses exist in an article, and especially when such sections share a common intersection. Nonuniform shrinkage often results in part distortion and decreased precision of the article.
Thirdly, the green article is relatively fragile and is therefore subject to breakage during demolding with consequent decreases in process yield. This is a significant problem with larger parts. The articles in their final, sintered state are also subject to breakage.
Accordingly, it is an object of the present invention to reduce the cost per unit volume of molded refractory articles.
It is another object of the present invention to reduce the handling of the parts during the various stages of production, thus reducing damage to the parts produced.
It is a further object of the present invention to establish a processing scheme which is readily adaptable to automated control.
It is a still further object to eliminate the need for the intermediate and interim RTV mold.
It is a further object of the invention to pursue the above and other objects, which will be apparent from the instant disclosure, alone and in various combinations.